Preheat the oven to 350 degrees.
Remove the top inch of the asparagus tips and split them lengthwise. Cut the remaining stalks crosswise into very thin slices.
Split the leek in half lengthwise and slice the white end and the light green part of the stalk into half moon shaped slices about 1/4 inch thick. (You will have about 1 1/2 cups.) Put the leek slices in a large bowl of water and swish them around to rinse well, changing the water until it runs clear. Lift them from the water with your hands or a slotted spoon and set aside.
Melt the butter in a 10 inch, oven safe, nonstick saute pan over medium heat until it begins to bubble. Redue the heat to medium low and cook the asparagus and leeks for about 10 minutes, or until the vegetables begin to soften. Season with salt and pepper and spread the vegetables evenly in the pan. Whisk together the eggs, half and half, and 1 tsp salt. Season lightly with pepper. Pour into the pan with the asparagus and leeks and sprinkle the feta over the top of the frittata. Cook over medium heat for about 1 minute before transferring the pan to the oven for 20 to 25 minutes, or until set in the center.